Normally edge detection devices are used to determine the position of a sheet-shaped print material on its way through a sheet-processing machine in order to be able to signal when sheet-shaped print material is present, for example at the start of a printing or further-processing procedure. Examples of sheet-shaped print material are printed or unprinted paper or transparencies.
The detection of a leading or trailing edge of such a sheet-shaped print material is especially important when several processing steps within the sheet-processing machine will have a fixed spatial relationship to the sheet-shaped print material, e.g. the printing of different color separations and/or precisely positioned placement of holes, folds or other processes relative to each other.
To detect the edge of a sheet-shaped print material, many different techniques are used, including the use of optical sensors. Optical sensors may be either reflection or transmission sensors, depending on whether transmitters and receivers are located on the same side of the sheet-shaped print material or on opposite sides of the sheet-shaped print material.
An example of such a system A special problem occurs in the detection of edges of transparencies. An example of attempts to overcome this problem can be found in U.S. Pat. No. 5,139,339, which is hereby incorporated by reference. In that patent, one transmitter and two receivers for beams are mounted above the path of the paper in such a way that the first receiver can only receive light that is reflected diffusely from the surface of the sheet-shaped print material. On the other hand, the second receiver essentially receives a beam that is reflected directly from the surface of the sheet-shaped print material according to linear beam optics. In the absence of a sheet-shaped print material, neither of the receivers receives a reflection. Because of this arrangement, a differentiation can be made between opaque surfaces (e.g. paper), which essentially scatter diffusely and thus mainly generate a signal at the first receiver, and transparent surfaces that mainly generate a signal at the second receiver.
U.S. Pat. No. 5,859,440, which is hereby incorporated by reference, discloses another principle for detection of edges of transparencies, which utilizes the shadow on an edge of a transparent material. If light is radiated at a flat or acute angle onto the edge of a transparent material, then total reflection occurs within the transparent material at the surface of the outer edge so that on the side of the edge opposite to the transmitter a shadowed area develops that can be detected by a suitable optical sensor. A shadow such as this also develops with opaque materials such that the sensor can also be used to detect edges of sheet-shaped print materials.
Japanese patent specification JP 62202206, which is hereby incorporated by reference, shows an optical device for detecting edges of sheet-shaped print materials, in which transmitters and receivers are located on different sides of the sheet-shaped print material. In this case, a broad light source is projected, by means of optics, on the other side of the sheet-shaped print material onto a wide detection line, which senses an exact position of the edge. In order to also detect edges of transparent sheet-shaped print materials, the optical axis between transmitter and receiver forms a flat or acute angle with the surface of the sheet-shaped print material.
Efforts regarding such systems have led to continuing developments to improve their efficiency, versatility, and practicality.